Williams, a three-time champion and seeded No.29, has won 54 matches on the island. She considers the tournament her home event because she lives an hour away in Palm Beach Gardens.

Her latest victory took two-and-a-half hours.

"I love being here," she said. "I wanted to play [in the next round]."

Williams' next opponent will be 10th seed Dominika Cibulkova, who beat No.22 Alize Cornet 7-6(6) 6-1. No.11 Caroline Wozniacki lost only five points in the second set as she routed 17th seed Sloane Stephens 6-1 6-0.

Li Na overcame three set points to overtake Madison Keys 7-6(3) 6-3. The No.2-seed and reigning Australian Open champion won even though she had seven double-faults and was broken four times.

Keys failed to convert her set points serving at 5-3 in the first set. She also squandered a lead in the second set, when three times she was within a point of going up 3-love against Li.

"She's No.2 in the world for a reason," Keys said. "She just won Australian Open for a reason. She's a great player. I played well at times, and she just played the bigger points really, really well."